This invention relates to diaphragm pumps, and more particularly to diaphragm pumps of the type suitable for use with engines for supplying secondary air thereto.
In one prior type of diaphragm pumps, movement of an eccentric cam on an engine is converted by a lever into reciprocatory movement which in turn is transmitted to a diaphragm of the pump through a shaft. This type of diaphragm pump is well known as fuel pump or vacuum pump. In this type of diaphragm pumps, the lever is urged at one end thereof against the eccentric cam by a spring and connected at the other end thereof to the shaft in such a manner that the lever exerts a one-way force on the shaft to push the latter either upwardly or downwardly, so that the diaphragm can be returned to its original position by the spring. When this type of diaphragm pump is used as fuel pump, the diaphragm is actuated by the spring in a discharge stroke to produce a predetermined discharge pressure of 0.2 to 0.3 kg/cm.sup.2 and discharge a small amount of fuel. When this type of diaphragm pump is used as air pump for supplying secondary air to the engine, a return spring for diaphragm in a single-acting diaphragm pump should have a setting load sufficient to cause the pump to produce a discharge pressure of 0.5 to 0.6 kg/cm.sup.2 and to deliver an amount of discharge ten times as much as the discharge of the fuel pump. Thus the force required for the drive lever to push or pull the shaft directly connected to the diaphragm is equal to or higher than the total pressure due to the discharge pressure plus the setting load of the return spring plus inertia. Therefore, the force for driving the driving lever is intermittent and very large to exert a high load on the engine. Also, a surface pressure produced between the diaphragm and the adjacent parts is large, resulting in reduced durability of diaphragm.
There have been proposed a double-acting type diaphragm pump having a large discharge at low revolution, being compact in size and having a single flat or pan-shaped diaphragm. In this type of diaphragm pump, the diaphragm moves repeatedly in the normal and reverse directions to produce noise, and is less durable. The maximum number of revolutions of the diaphragm pump is limited to about 2000 rpm., so that it is impossible to supply a required discharge of secondary air.